There are many applications wherein structures are subjected to loads which, over time, have a tendency to fatigue the material and create a risk of failure. Thus, it is highly desirable to be able to test these structures to determine the remaining fatigue life such that they might be replaced or renewed prior to failure. In still other instances, and especially for critical applications involving health and safety, standards have been established for the routine testing of certain parts prior to their being placed in service to ensure against failure of the part. In those applications, techniques have been developed and are available in the prior art to achieve such testing. These techniques include radiographic inspection, florescent penetrant inspection, destructive testing of selected parts from a lot, and other techniques, all of which are well known in the art. However, these techniques are all subject to certain drawbacks such as expense, inconvenience, and in some cases failure to entirely eliminate or predict the premature failure of the part.
Still another situation in which these kinds of tests are conducted involve many instances where materials or parts are welded and the integrity of the weld must be verified prior to the equipment being placed in service. One particular application, from amongst many, involves the federal safety standards which govern the construction of nuclear power plants. Certain welds in certain critical equipment contained within the plant are subjected to radiographic inspection and other kinds of testing in order to verify their integrity prior to the plant being placed in service. A nuclear power plant presents perhaps an extreme example of the potential harm which might befall not only the people involved but the public at large should a critical piece of equipment suffer a premature failure. There are a myriad of other applications perhaps considered not as critical but which also are important to the health and safety of many people, including the public at large.
Because the various types of testing described above are used to predict various types of failure modes involving crack initiation and propagation other than fatigue, these testing techniques will be referred to collectively as structural integrity testing. The resulting determinations made through structural integrity testing include the amount of part life used in units of time or cycles, the amount of part life remaining in time or cycles, and the size of the largest characteristic flaw. As appreciated by those in the art, these quantities aid in the analysis and management of the structures being analyzed. For instance, an aircraft fleet operator may perform a cost-benefit analysis to determine whether a particular part should be retired or returned to service.
Despite the fact that structural integrity testing has been used for some time, and the relationship of damping to fatigue has been well known for some time, the inventor is not aware of any other efforts in the prior art to utilize the relationship between damping and fatigue in the arenas of predicting failure and determining structural integrity. For example, in a paper presented at a colloquium on structural damping at the American Society of Mechanical Engineers (A.S.M.E.) Annual Meeting in December 1959, the phenomenon of plastic strain was analyzed. In particular, damping was found to be a function of stress history and stress amplitude. As concluded in the paper, at low stresses and intermediate stresses, i.e. stresses below fifty percent of the fatigue limit, damping was not seen to be affected by the stress history of the material. On the other hand, at high stresses, i.e. stresses above fifty percent of the fatigue limit, where large plastic strain damping may be observed, stress history played a part in affecting plastic strain, as measured by the modal damping factor. Stated differently, data were presented indicating that at low and intermediate stress, the modal damping factor does not change with the number of fatigue cycles. However, above a critical stress, damping increases with the number of fatigue cycles thereby indicating that stress history and stress amplitude play a part in modal damping factor under these conditions. Although this article treated the interrelationship between stress history and stress amplitude, and their effect on damping, there was no disclosure or suggestion of using a measured modal damping factor as an indicator of structural integrity. As stated therein, the article focused on how stress history and amplitude might produce a particular modal damping factor but not how a measured modal damping factor could be used as a predictor of relative fatigue in a part. See Structural Damping, A.S.M.E. Proceedings, (Jerome E. Ruzicka ed., 1959).
In order to solve these and other problems in the prior art, and as a departure from the teachings in the prior art, the inventor herein has succeeded in developing the technique of measuring the modal damping factor of a discrete portion of a structure, such as a part in an assembly or the like, and using that modal damping factor for determining the structural integrity of that part either by comparing it with a standardized modal damping factor or with previously measured modal damping factors for the same part. The part might be a single piece of material, or it might be a welded or otherwise joined piece of material and the test may be one for integrity as might be required for a new part, or the test might be conducted for determining the fatigue in the part after having been installed and used over time. For new part testing, it is anticipated that standardized modal damping factors may be determined and available for comparison with the measured modal damping factor for the new part. Alternately, the modal damping factor of a series of identical new parts might be measured and used to cull out those new parts which evidence signs of manufactured flaws such as cracks, voids, or other defects. After a part has been installed and used over a period of time, a modal damping factor measurement may be made periodically as an indication of the level of fatigue the part has undergone. This technique may be used to identify parts which are in need of replacement prior to any chance of failure. In addition, there are other applications and situations in which the modal damping factor measurement of a structure might be used to good advantage. Thus, these particular examples are given as exemplary and are not intended to limit the scope of the invention described herein.
In making the modal damping factor measurement, the inventor has also succeeded in developing a simple but effective and accurate technique for measuring the modal damping factor using either of two methods. In the first method, an impulse of energy is applied to the part, such as by striking the part with a blunt object or the like, and the induced vibration in the part is measured by a transducer which converts the vibration into an electrical signal for input to a computer. A computer may used to make the appropriate calculations from the induced vibration to determine the modal damping factor as is well known in the art. Generally, the modal damping factor of a part vibrating at its natural frequency from an impulse input may be determined by comparing peak displacement amplitudes of successive cycles of the vibration. In the second method, a continuous stream of energy is input to the part instead of an impulse of energy. In a preferred embodiment, a frequency generator and amplifier may be coupled to a transducer, such as a speaker, shaker, or other such device, and the frequency generator tuned or adjusted so as to sweep through the range of the lowest natural frequencies of the part. As the frequency of the input is changed, the peak displacement amplitude of the part will vary. The modal damping factor may be readily calculated by measuring the half-power bandwidth of a cycle of displacement and dividing it by the displacement amplitude at the center frequency, as will be explained in greater detail below. Using either of these methods, a vibration is induced in the part and is measured to determine the modal damping factor.
Digital computer analysis techniques typically sample data at time intervals. This discrete sampling presents inaccuracies and computational difficulties in analyzing the part response as is well known in the art and further compounds the analysis difficulties due to noise. In order to solve these problems caused by discrete sampling, the inventor has succeeded in developing a computer algorithm which estimates the modal damping factor from discrete vibration data received from the part. The algorithm matches the measured response with a theoretical one degree of freedom system response and varies the theoretical system parameters until a suitable correlation between the theoretical and actual responses is achieved. When the suitable correlation is achieved, the actual part modal damping factor is estimated to be that of the theoretical system.
One of the advantages of using the inventor's method of inducing a vibration in the part is that it is believed that the part need not be isolated and may be tested in place within an assembly or other structure. This eliminates disassembly of the part from any larger assemblage which dramatically reduces any cost involved in using the present method in determining the modal damping factor. This provides great advantages over other prior art methods which require disassembly and isolation of the part to be tested, such as during most radiographic inspection. Furthermore, the device used to implement the method disclosed herein may be relatively compact, readily portable, and sufficiently small such that the testing of many different parts which might be otherwise relatively difficult to access may be readily achieved.
While the principal advantages and features of the present invention have been described above, a more complete and thorough understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follow.